Smoke Simulation is different in 3D viewport from final render
Although the quick smoke feature creates the texture nodes for the smoke, it doesn't for the flames.  So when you render it, only the smoke is visible.

This has been covered before on Blender StackExchange:

https://blender.stackexchange.com/questions/55777/fire-showing-in-viewport-not-in-render

This is also covered in Blender Guru's fire tutorial:

Blender Guru - Fire